Imaging technology is the science of converting an image to a signal indicative thereof. Imaging systems have broad applications in many fields, including commercial, consumer, industrial, medical, defense and scientific markets.
The original image sensors included an array of photosensitive elements in series with switching elements. Each photosensitive element received an image of a portion of the scene being imaged. That portion is called a picture element or pixel. The image obtaining elements produce an electrical signal indicative of the image plus a noise component. Various techniques have been used in the art to minimize the noise, to thereby produce an output signal that closely follows the image.
Size minimization is also important. The development of the solid state charge coupled device (“CCD”) in the early 1970's led to more compact image systems. CCDs use a process of repeated lateral transfer of charge in an MOS electrode-based analog shift register. Photo-generated signal electrons are read after they are shifted into appropriate positions. However, the shifting process requires high fidelity and low loss. A specialized semiconductor fabrication process was used to obtain these characteristics.
CCDs are mostly capacitive devices and hence dissipate very little power. The major power dissipation in a CCD system is from the support electronics. One reason for this problem is because of the realities of forming a CCD system.
The specialized semiconductor fabrication process alluded to above is not generally CMOS compatible. Hence, the support circuitry for such a CCD has been formed using control electronics which were not generally CMOS compatible. The control electronics have dissipated an inordinate percentage of the power in such imaging devices. For example, CCD-based camcorder imaging systems typically operate for an hour on an 1800 mA-hr 6 V NiCad rechargeable battery, corresponding to 10.8 W of power consumption. Approximately 8 watts of this is dissipated in the imaging system. The rest is used by the tape recording system, display, and autofocus servos.
Space-based imaging systems often have similar problems. The space based systems operate at lower pixel rates, but with a lower degree of integration, and typically dissipate 20 watts or more.
The CCD has many characteristics which cause it to act like a chip-sized MOS capacitor. The large capacitance of the MOS device, for example, requires large clock swings, ΔV, of the order of 5-15 V to achieve high charge transfer efficiency. The clock drive electronics dissipation is proportional to CΔV2f, and hence becomes large. In addition, the need for various CCD clocking voltages (e.g. 7 or more different voltage levels) leads to numerous power supplies with their attendant inefficiencies in conversion.
Signal chain electronics that perform correlated double sampling (“CDS”) for noise reduction and amplification, and especially analog to digital converters (ADC), also dissipate significant power.
The inventors also noted other inefficiencies in imaging systems. These inefficiencies included fill factor inefficiencies, fixed pattern noise, clock pick up, temporal noise and large pixel size.
Active pixel sensors, such as described in U.S. Pat. No. 5,471,515, the disclosure of which is incorporated by reference herein, use special techniques to integrate both the photodetector and the readout amplifier into the pixel area or adjacent the pixel area. This allows the signal indicative of the pixel to be read out directly. These techniques have enabled use of a logic family whose fabrication processes are compatible with CMOS. This has enabled the controlling circuitry to be made from CMOS or some other low power-dissipating logic family.
The inventors of the present invention have recognized techniques and special efficiencies that are obtained by specialized support electronics that are integrated onto the same substrate as the photosensitive element. Aspects of the present invention include integration, timing, control electronics, signal chain electronics, A/D conversion, and other important control systems integrated on the same substrate as the photosensitive element.
It is hence an object of the present invention to provide for the integration of an entire imaging system on a chip.